Fans a large / unbounded notification audience out to per-recipient rows off the request path.
Enqueued by MishkaCmsCore.Notify.Delivery for :permission / :masters (or large explicit)
audiences. It re-resolves the audience from the serialized args and STREAMS recipients in
batches via Delivery.fan_out/4, so a site with a very large user base is written in constant
memory. unique (on dedup_key) collapses accidental duplicate fan-outs. The site is carried
in the args ("site_id") and resolved here — never re-derived.